The present invention generally relates to a tungsten deposition process that incorporates a nucleation step and more particularly, relates to a tungsten deposition process for filling via openings in a semiconductor substrate incorporating a dual-step nucleation process for depositing first a WSix layer and then a W layer on top.
In the fabrication of semiconductor devices, metal contacts and vias are frequently formed in contact holes and via openings on silicon wafers that have been pre-processed with insulating layers on top. Devices are then fabricated by connecting the components with metal contacts and vias to form the integrated circuit. In particular, aluminum, aluminum alloys, tungsten and tungsten alloys are used for depositing into contact holes and via openings on silicon substrates. The deposition process can be carried out either in a physical vapor deposition (PVD) chamber or in a chemical vapor deposition (CVD) chamber.
As the dimensions of semiconductor devices continuously to shrink in the miniaturization of modern semiconductor devices to the sub-half-micron range, via openings and contact holes must also shrink. Consequently, the openings and holes to be filled have larger aspect ratios, i.e., the ratios between the depth of the opening or hole and the diameter.
Difficulties have been encountering in depositing conductive metals into via openings and contact holes that have high aspect ratios by the conventional sputtering process. As the openings or holes become smaller and deeper, the bottom and sides of an opening or hole receive fewer deposited metal particles than the top surface of the device. The end result of such a phenomenon, sometimes called a shadowing effect, is that metal layers formed by the particles hang over the opening forming an overhang. The overhand closes before the opening is completely filled as the deposition process progresses and thus creating a void in the opening or hole.
One technique used to remedy the shadowing effect of the sputtering process is to use a tungsten chemical vapor deposition (W CVD) technique for filling openings and holes that have large aspect ratios. The W CVD process solves the difficult problems in metalization to ensure enough metal continuity in contact windows and vias. The step coverage of deep openings or holes by the W CVD particles is greatly improved over that possible by any other deposition techniques. In a W CVD process conducted on a silicon substrate for filling a contact hole, the basic chemistry may be a reaction such as
2WF6+3SiH4xe2x86x922W+3SiF4.
During a W CVD deposition process, a wafer is usually held on a vacuum chuck that is heated to a temperature between about 400xc2x0 C. and about 500xc2x0 C. A shower head is positioned opposite to the wafer where WF6/SiH4 gases are injected. Normally, a two-or three-step process is involved where SiH4 is first introduced without any flow of WF6 to initiate a deposition of a very thin seed layer of amorphous silicon as a prenucleation layer. The prenucleation process is then followed by a SiH4+WF6 silane reduction nucleation process for depositing a thin W nucleation layer, and then the faster rate H2+WF6 hydrogen reduction process for bulk W deposition. During the nucleation stage, less than 100 nm of tungsten is deposited, while the bulk of the tungsten deposition is by the hydrogen reduction process. The multi-stage deposition process is designed such that during the initial nucleation stage, the silicon from the source/drain area is not consumed in the reaction since WF6 would react readily with Si. When WF6 reacts with Si from the source/drain region, a defect known as junction leakage may occur. The introduction of SiH4 first into the reaction avoids the consumption of Si from the substrate. The initial introduction of SiH4 into the reaction without WF6 for the deposition of the prenucleation layer of Si is known as a silane soak step.
In the W CVD process, a W CVD is frequently blanket-deposited onto a wafer surface and into the contact holes after a metal nucleation layer is first deposited on the entire wafer. The W deposited on the glue/barrier layer, i.e., a Ti/TiN layer, is then etched off in an etchback process by a process of reactive ion etching or by CMP process. After the etchback process, only the thicker W in the contact holes are left. Since the process relies on the removal of all W CVD except in the contact holes, the uniformity of the W deposition and the RIE etchback process is critical for the successful formation of W contact plugs. When the process is not accurately controlled, such as with the pre-disposition of a nucleation layer of Ti/TiN, the W contact plugs may be substantially recessed after the etchback process and thus results in poor step coverage on the device.
In the conventional single-step nucleation process conducted after a diffusion barrier layer deposition, the ratio of WF6/SiH4 in the reactant gas mixture must be kept low in order to avoid chemical attack of WF6 or defect formation known as volcano effect. At a low flow rate of WF6, the step coverage of the subsequent tungsten deposition for forming the plug is poor. The step coverage problem becomes more severe with decreasing feature size at the contact or via level which renders a perfect plug forming impossible. The step coverage in filling a contact of via is extremely important in a tungsten CVD process in order to achieve desirable resistance values and device reliability. The conventional single step nucleation process is therefore inadequate for achieving high reliability plug filling and for improving step coverage.
It is therefore an object of the present invention to provide a tungsten plug deposition process that does not have the drawbacks or shortcomings of the conventional tungsten deposition process incorporating single-step nucleation.
It is another object of the present invention to provide a tungsten plug deposition process incorporating a dual-step nucleation method.
It is a further object of the present invention to provide a tungsten plug deposition process incorporating a dual-step nucleation method such that two separate nucleation layers are deposited into via openings before the bulk tungsten deposition.
It is another further object of the present invention to provide a tungsten plug deposition process incorporating a dual-step nucleation method by using a reactant gas mixture of WF6/SiH4 at different mix ratios for the two nucleation layers.
It is still another object of the present invention to provide a tungsten plug deposition process incorporating a dual-step nucleation method in which a first nucleation step is carried out by flowing a reactant gas mixture that is silicon rich followed by a second nucleation step by flowing a reactant gas mixture that is tungsten rich.
It is yet another object of the present invention to provide a tungsten plug deposition process incorporation a dual-step nucleation method by forming two separate nucleation layers consisting of a first layer of WSix and a second layer of W.
It is still another further object of the present invention to provide a semiconductor structure that has tungsten plugs formed in a top surface including, in each plug, a first nucleation layer contacting the via openings of substantially WSix and a second nucleation layer on top of the first nucleation layer of substantially W.
It is yet another further object of the present invention to provide a method for tungsten plug deposition incorporating a dual-step nucleation process in which a first nucleation layer is deposited by a reactant gas mixture of WF6/SiH4 having a mix ratio smaller than 1:1 followed by the deposition of a second nucleation layer by flowing a reactant gas mixture of WF6/SiH4 having a mix ratio larger than 2:1.
In accordance with the present invention, a tungsten plug deposition process incorporating a dual-step nucleation method and the semiconductor structure formed with tungsten plugs are disclosed.
In a preferred embodiment, a tungsten plug deposition process incorporating a dual-step nucleation method can be carried out by the operating steps of positioning a substrate that has via holes for forming tungsten plugs therein in a chemical vapor deposition (CVD) chamber, depositing into the via holes a first nucleation layer by flowing a reactant gas mixture of WF6/SiH4 at a first mix ratio between 1:1 and 1:10 into the CVD chamber, depositing into the via holes a second nucleation layer on top of the first nucleation layer by flowing a reactant gas mixture of WF6/SiH4 at a second mix ratio between 2:1 and 5:1 into the CVD chamber, and filling the via holes with tungsten forming the tungsten plugs.
The method for forming tungsten plugs incorporating a dual-step nucleation method may further include the step of depositing the first and the second nucleation layer to a total thickness of less than 500 xc3x85. The method may further include the step of depositing the first nucleation layer by flowing a reactant gas mixture of WF6/SiH4 at a first mix ratio between 1:1 and 1:5. The method may further include the step for depositing a second nucleation layer by flowing a reactant gas mixture of WF6/SiH4 at a second mix ratio between 2:1 and 5:1. The method may further include the step of depositing the first and the second nucleation layers at a temperature between about 380xc2x0 C. and about 450xc2x0 C., or preferably between about 410xc2x0 C. and about 430xc2x0 C. The method may further include the step of flowing the reactant gas mixture of WF6/SiH4 into the CVD chamber to a chamber pressure between about 4.5 Torr and about 30 Torr. The method may further include the step of flowing the reactant gas mixture of WF6/SiH4 at a first mix ratio into the CVD chamber for a time period between about 3 sec and about 10 sec, or the step of flowing the reactant gas mixture of WF6/SiH4 at a second mix ratio into the CVD chamber for a time period between about 10 sec and about 25 sec. The first nucleation layer deposited may be substantially WSix, the second nucleation layer deposited may be substantially W.
The present invention is further directed to a method for forming tungsten plugs incorporating a dual-step nucleation process which can be carried out by the operating steps of positioning a substrate in a chemical wafer deposition chamber, the substrate has via openings formed in a top surface, depositing into the via openings a first nucleation layer of WSix, depositing into the via openings a second nucleation layer of W on top of the first nucleation layer and filling the via openings with W forming the W plugs.
The method for forming tungsten plugs incorporating a dual-step nucleation method may further include the step of depositing the first and the second nucleation layer to a total thickness between about 200 xc3x85 and about 500 xc3x85. The method may further include the step of depositing the first and the second nucleation layer by a reactant gas mixture of WF6/SiH4. The method may further include the step of depositing the first nucleation layer by a reactant gas mixture that contains more SiH4 than WF6, or the step of depositing the second nucleation layer by a reactant gas mixture that contains more WF6 than SiH4. The method may further include the step of depositing the first nucleation layer for a shorter length of time than the time required for depositing the second nucleation layer.
The present invention is further directed to a semiconductor structure that has tungsten plugs formed in a top surface including a semiconductor substrate that has a top active surface, a plurality of via openings in the top active surface, and a plurality of tungsten plugs formed in the plurality of via openings each further includes a first nucleation layer contacting surfaces in the plurality of via openings formed of substantially WSix, a second nucleation layer on top of the first nucleation layer formed of substantially W, and a tungsten filling the plurality of via openings.
In the semiconductor structure that has tungsten plugs formed in a top surface, the first nucleation layer and the second nucleation layer may have a total thickness between about 200 xc3x85 and about 500 xc3x85. The semiconductor structure may further include a diffusion barrier layer deposited between the first nucleation layer and the inner wall of the plurality of via openings. The semiconductor structure may be a silicon wafer.